Video imaging systems represent an input scene as a time-varying signal that can be transmitted or stored by electronic, magnetic, optical, or other means. Video is gathered as a succession of "still" frames representing the input scene, at a frame rate such that, when the frames are subsequently presented in rapid sequence to a human observer, motion appears fluid. FIG. 1 shows, with a great deal of simplification, one method of creating and replaying a video signal frame 20, known as progressive video. In this method, the video time signal represents the image intensity at a raster point scanning left to right, top to bottom across frame 20 at a constant rate, tracing out line pattern 1-8 during one frame time. At the end of this frame time, the point has imaged a frame, and it returns to the beginning of line 1 and begins a new frame.
Modern video imaging systems use a variety of "tricks" to minimize video bandwidth requirements while presenting acceptable picture quality. One trick that is used to reduce detectable screen flicker produced by raster scanning a cathode ray tube at a low frame rate is interlaced video. The current United Stated video standard, commonly referred to as NTSC (National Television System Committee) video, uses interlacing.
FIG. 2 illustrates an interlaced composition of frame 22. Like frame 20, six full-scan and two half-scan lines make up frame 22. Unlike frame 20, the scan lines are not gathered in one pass top to bottom. Instead, interlaced frame 22 gathers half the lines (lines 1-4) in a first pass top to bottom, and the other half (lines 5-8) in a second pass.
Each interlaced scan pass is called afield. Two adjacent-in-time fields 24 and 26 trace out every line in frame 22. These fields are commonly referred to as top and bottom. For purposes of this disclosure, the top field is defined to contain the topmost full scan line of the frame--generally, the naming convention is not important to system implementation.
If motion exists in an input scene, an interlaced frame 22 will not contain the same information as a progressive frame 20 gathered for the same input scene at the same frame rate. This fact is evident from a comparison of FIGS. 1 and 2. For example, scan line 1 is gathered first in FIG. 1; in FIG. 2, its corresponding scan line 5 is gathered fifth, or half a frame later. If the scene changes during this interval, the progressive and interlaced data will differ.
In some applications, it is desirable to convert interlaced video to progressive video. For example, some large-format video display devices use progressive format to enhance apparent picture quality, and some digital systems prefer non-interlaced image presentation. Typical interlace-to-progressive video conversion employs line doubling, where two adjacent progressive scan lines are output during a single interlaced video scan line period. The first of the two scan lines is taken from the top interlaced field, and the second is taken from the bottom interlaced field. This method doubles the frame rate of the progressive video, as compared to the frame rate of the interlaced video, although no additional information is added.
FIG. 3 depicts an interlaced to progressive conversion process. An interlace video sequence is shown having four top fields A, C, E, G and three bottom fields B, D, F. Each line-doubled progressive frame is created by combining a top field with an adjacent bottom field as shown in FIG. 3. For example, bottom field B is first combined with top field A to create progressive video frame AB. When top field C arrives, B may then be combined with C to create a second progressive video frame CB.
In order to create progressive video in this manner, one of the two fields always must be delayed by one field time in order to allow combination with the present field. Converter circuit 28 of FIG. 4 accomplishes this using a field store 32. Video input 30 is delayed in field store 32 for one field time, allowing each field in input 30 to be used during the generation of two consecutive progressive frames. A field sync line controls top switch 34 and bottom switch 36, such that a top field will always be routed to top field line store 38, and a bottom field will always be routed to bottom field line store 40. Line stores 38 and 40 are controlled by a line sync signal, such that line-doubled progressive video is output correctly.
Motion picture film and video are typically captured at different frame rates. Most motion picture film is captured at a frame exposure rate of 24 frames per second (i.e., a 24 Hz frame rate). In the United States, video typically uses a capture rate of 59.94 interlaced fields per second (for purposes of this disclosure, this rate is rounded and referred to as a 60 field-per-second rate, or a 60 Hz field rate). Since two interlaced fields are required to paint an entire frame, the frame rate of 60 Hz interlaced video is 30 Hz.
Because motion picture film and video frame rates differ, a motion picture film cannot be distributed by televised or recorded video without rate conversion. The most common method for converting 24 Hz frame rate motion picture film to 60 Hz field rate video is termed 3:2 pulldown.
The basic concept of 3:2 pulldown is shown in FIG. 5. Half of the motion picture frames (e.g., frames A and C in FIG. 5) are transferred as three interlaced video fields, and half of the motion picture frames (e.g., frames B and D) are transferred as two interlaced video fields. Three and two field transfers are alternated, resulting in a 3, 2, 3, 2, 3, 2 pattern that transfers every two motion picture frames using five video fields. After four film frames (and ten video fields), the pulldown pattern repeats. Note that for film frame A, two top video fields and one bottom video field are produced, while for film frame C, two bottom video fields and one top video field are produced.
Interlace-to-progressive converter 28 of FIG. 4 produces conversion artifacts if the video signal presented to it is a 3:2 pulldown signal. FIG. 6 shows the 3:2 pulldown video pattern of FIG. 5, along with a progressive conversion produced by converter 28. In the pulldown video signal, motion between successive fields does not occur over 1/60.sup.th of a second intervals like it does for conventional video. Instead, successive fields either have no motion (because they were created from the same film frame) or exhibit motion at the film frame rate of 1/24.sup.th of a second. Mismatches in the progressive conversion of 3:2 pulldown occur when fields are combined that originated in separate film frames. In FIG. 6, these are identified as progressive frames A2/B3, B4/C5, D8/C7, and E10/D9. The frame mismatch produces undesirable conversion artifacts.
Yves Faroudja recognized that by modifying an interlace-to-progressive video converter, the creation of these conversion artifacts could be avoided. In U.S. Pat. No. 4,876,596, issued to Faroudja on Oct. 24, 1989, and entitled "Film-to-Video Converter With Scan Line Doubling", a system is disclosed that avoids the creation of motion artifacts during line doubling of 3:2 pulldown material. This system is shown in FIG. 7.
Faroudja envisioned a modified NTSC system that provided an indication of the presence of 3:2 pulldown material within the NTSC signal itself. In Faroujda's converter system 46, this modified NTSC video was fed to a specialized NTSC decoder 42 that produced red, green, and blue (R, G, and B) interlaced video channels, and a sequence control signal that indicated where in the processing of the ten-field repeating pattern of 3:2 pulldown the system currently was. Each of the RGB channels was fed to a separate interlace-to-progressive converter 44. Each converter 44 maintained copies of the two previous fields. By appropriate sequencing in multiplexer 60, the current field and the two previous fields were combined into a progressive frame at each field time.
FIG. 8 illustrates the conversion process disclosed in the '596 patent. At field time 1, top field A0 and bottom field A1 are combined to form a progressive frame A0/A1. At field time 2, top field A2 and bottom field A1 are combined to form a progressive frame A2/A1. These two steps are identical to normal progressive frame formation as shown in FIG. 3. But at frame time 3, instead of combining top field A2 with bottom field B3, the combination used in frame 2 is repeated. This requires converter 44's additional field store 50, which allows bottom field A2 to be delayed for an additional field time. The resulting progressive frame sequence contains no mismatched fields, and results in an improved picture quality for 3:2 pulldown video.
In U.S. Pat. No. 4,881,125, issued to Edward Krause on Nov. 14, 1989, and entitled "Progressive Scan Display of Video Derived From Film", a method for implementing a noise-averaging line-doubling system for 3:2 pulldown material was disclosed. The '125 method required a two-field delay, as illustrated in FIG. 9. By waiting two field times, this method allows all three "A" fields from the 3:2 pulldown video signal to be gathered. The method then averages the identical (except for noise) repeated top "A" fields A0 and A2 to obtain a 3 dB noise-reduced top field A0/2. Three identical progressive frames A0/2/A1 are then output at field times 2, 3, and 4. This method requires an additional field store and an averager for each channel, and introduces an extra field of delay as compared to the '596 patent. In return, a 3 dB noise reduction may be observed on half of the scan lines of the progressive video output, for three out of every five field times.